Fossil resources, such as oil cuts, for the chemical industry will be exhausted in a few decades. Resources of natural and renewable origin such as alternative raw materials are consequently being studied more and more.
Acrolein is a key intermediate for the synthesis of methionine, a synthetic protein used as an animal feed supplement, which has emerged as a substitute for fishmeal. Acrolein is also a non-isolated synthetic intermediate of acrylic acid, for which the importance of its applications and its derivatives is known. Acrolein also leads, via reaction with methyl vinyl ether then hydrolysis, to glutaraldehyde, which has many uses in leather tanning, as a biocide in oil well drilling and during the treatment of cutting oils, and as a chemical sterilising agent and disinfectant for hospital equipment.
Acrolein is produced industrially by oxidation, in the gas phase, of propylene via the oxygen in the air in the presence of catalyst systems based on mixed oxides. Glycerol, derived from plant oils in the production of biodiesel fuels is one of the routes envisaged as a substitute for propylene, glycerol being able to be subjected to a catalytic dehydration reaction in order to produce acrolein. Such a process makes it possible to thus respond to the concept of green chemistry within a more general context of protecting the environment.
Numerous catalyst systems have already been the subject of studies for the dehydration reaction of glycerol to acrolein.
A process is known from Patent FR 695 931 for preparing acrolein from glycerol according to which acid salts having at least three acid functional groups or mixtures of these salts are used as catalysts. The preparation of these catalysts consists in impregnating, for example with iron phosphate, pumice that has been reduced to pea-sized fragments. According to the teaching of the patent, the yield obtained with this type of catalyst is greater than 80%.
In U.S. Pat. No. 2,558,520, the dehydration reaction is carried out in gas/liquid phase in the presence of diatomaceous earths impregnated with phosphoric acid salts, in suspension in an aromatic solvent. A degree of conversion of glycerol to acrolein of 72.3% is obtained under these conditions.
U.S. Pat. No. 5,387,720 describes a process for producing acrolein by dehydration of glycerol, in liquid phase or in gas phase, at a temperature ranging up to 340° C., over solid acid catalysts that are defined by their Hammett acidity. The catalysts must have a Hammett acidity below +2 and preferably below −3. These catalysts correspond, for example, to natural or synthetic siliceous materials such as mordenite, montmorillonite or acidic zeolites; supports, such as oxides or siliceous materials, for example alumina (Al2O3) or titanium oxide (TiO2) covered by monobasic, dibasic or tribasic inorganic acids; oxides or mixed oxides such as gamma-alumina, ZnO/Al2O3 mixed oxide, or else heteropolyacids. The use of these catalysts will make it possible to solve the problem of formation of secondary products generated with the iron phosphate type catalysts described in the aforementioned document FR 695 931.
According to Application WO 2006/087084, the strongly acidic solid catalysts for which the Hammett acidity H0 is between −9 and −18, have a strong catalytic activity for the dehydration reaction of glycerol to acrolein and are deactivated less quickly.
EP 2 006 273 discloses a process for production of acrolein from glycerol using aluminium phosphates with different aluminium to phosphorous ratios; the presence of an supplemental element in the structure of these catalysts is not contemplated.
Alumina impregnated by phosphoric acid are already known by FR 2 882 052 for the reaction of dehydration of glycerol, but rapid deactivation is observed for these catalysts.
However, the catalysts recommended in the prior art for producing acrolein from glycerol generally lead to formation of by-products such as hydroxypropanone, propanaldehyde, acetaldehyde, acetone, addition products of acrolein to glycerol, polycondensation products of glycerol, cyclic glycerol ethers, but also phenol and polyaromatic compounds that originate from the formation of coke on the catalyst and therefore from its deactivation. The presence of by-products in acrolein, especially propanaldehyde or propionic acid, poses numerous problems for the separation of acrolein and requires separation and purification steps that result in high costs for the recovery of purified acrolein. Furthermore, when acrolein is used for producing acrylic acid, the propanaldehyde present may be oxidized to propionic acid which is difficult to separate from acrylic acid, especially by distillation. These impurities that are present greatly reduce the field of application of the acrolein produced by dehydration of glycerol.
The Applicant Company has therefore sought to improve the production of acrolein from glycerol using more selective catalysts that make it possible to obtain high yields of acrolein and that have an activity over long periods.
In the field of catalysts, the use of catalyst systems based on iron phosphate has been widely described, these catalysts being particularly suitable for the oxydehydrogenation of saturated carboxylic acids to unsaturated carboxylic acids, in particular the conversion of isobutyric acid to methacrylic acid (FR 2,514,756; FR 2,497,795; FR 245,604; U.S. Pat. No. 4,364,856; FR 2,657,792; FR 2,498,475), or for the oxydehydrogenation of saturated aldehydes to unsaturated aldehydes and more specifically for the production of methacrolein from isobutaraldehyde (U.S. Pat. No. 4,381,411).
Mixed vanadium-phosphorus oxides are well known as catalysts for the selective oxidation of butane to maleic anhydride. The P/V atomic ratio of the active structure of these catalysts is generally between 1.2 and 2.0. Catalysts based on phosphorus and on vanadium with a P/V ratio between 1.0 and 1.2 have been found to be very effective, moreover, for the selective oxidation of butane to maleic anhydride (Bull. Chem. Soc. Jpn, 58, 2163-2171 (1985)).
The structure and the role of phosphorus in mixed V/P catalysts and also the reaction mechanisms used have been the subject of numerous studies. Among the active phases, mention may be made of VOHPO4.0.5H2O; VOPO4; VOPO4.2H2O; (VO)2P2O7; VO(H2PO4)2; VO(PO3)2. Various modes for preparing these catalysts may be used, among which mention may be made of:                a first mode consists in preparing a precursor, generally from vanadium oxide V2O5 and an acid solution such as, for example, oxalic acid, ammonium hydrogen phosphate or phosphoric acid. A precipitate is formed, which is then filtered, washed and dried. These precursors are then calcined under oxygen, nitrogen or air at a temperature generally between 450° C. and 600° C. (Okuhara et al., Bull. Chem. Soc. Jpn, 58, 2163-2171 (1985));        a second preparation mode is based on the preparation of the precursor VOHPO4.0.5H2O, then its thermal conversion to a vanadium pyrophosphate (VO)2P2O7 active phase (E. Bordes et al., Journal of Solid State Chemistry 55, 270-279 (1984); J. Johnson et al., J. Am. Chem. Soc. (1984), 106, 8123-8128); Busca et al., Journal of Catalysis, 99, 400-414 (1986));        a third preparation mode, which has been widely studied, is based on the preparation of the precursor VOPO4.2H2O, then its reduction with a primary or secondary alcohol such as, for example, 1-butanol, 2-butanol, isobutanol, pentanol, 2-pentanol, 1-, 2- or 3-hexanol, 1-, 2- or 3-heptanol, 1-, 2- or 3-octanol, nonanol, decanol, etc. (Hutchings et al., Catalysis Today, 33, (1997), 161-171); J. Chem. Soc. Chem. Commun. 1994, 1093-1094; J. Chem. Soc. Faraday Trans. 1996, 92(1), 137-142; Chemistry letters 2001, 184-485; Chem. Mater 2002, 14, 3882-3888).        
The catalytic activity of these various phases has been studied in the oxidation reactions of butane, of butene or of butadiene to maleic anhydride (G. Centi New developments in Selective Oxidation, 1990, Elsevier Science Publishers, B.V. Amsterdam, 605-615).
Other applications have also been found for mixed vanadium/phosphate oxides such as the ammoxidation of propane (G. Centi et al., J. Catal; 142 (1993), 70), the oxidation of pentane to phthalic anhydride (G. Centi et al., Sci. Technol. 1 (1995) 225). But these catalysts, just like other mixed oxides such as boron/phosphate or aluminium/phosphate, have never been the subject of studies for the dehydration reaction of glycerol to acrolein.
The Applicant Company has now found that systems based on a mixed oxide of phosphorus and on at least one metal chosen from vanadium, boron or aluminium have a high-performance catalytic activity for the dehydration of glycerol to acrolein while overcoming the drawbacks of the existing catalysts for this reaction.